Interface stability during the oxidation of binary alloys

The stability of a planar alloy/scale interface during the diffusion-controlled oxidation of a homogeneous, single-phase binary alloy depends on both the thermodynamic and transport properties of the system under consideration. A criterion is presented that can be employed to predict the stability of a planar alloy/scale interface for the specified reaction temperature, alloy composition, and chemical potential of the oxidant when only one component of the alloy is oxidized, anion diffusion predominates in the scale, and the solubility of oxygen in the alloy is essentially zero. A planar alloy/scale interface (a single-phase scale) is the preferred growth morphology if diffusion in the oxide phase is the rate-limiting step of the oxidation reaction. An uneven alloy/scale interface (a two-phase scale) is expected if diffusion in the alloy phase is the rate-determining step. This stability criterion is equivalent to the criterion derived by Wagner for the case of predominant cation diffusion in the scale